Adding a sub to a factory spec system.

I have a 2000 ford focuse zx3. i am thinking of adding a sub to it. The question i have is should i replace the other speakers in the car if i add a good sub to it. The factory system is a good system for what it is im very happy, just want some extra loudness and some bass. thanks.

You will want to probably get a new head unit first.
One when you connecting your amp you'll have to use a high/low converter or *shutters* high level inputs. SO getting a new head unit should beat out getting new inside speakers

ONce you have the money though, yes get new speakers, all it can do is improve you sound and immaging

yo the guy above is right .. but if you dont change your head unit you better get a dang good crossover, reason for that is, some factory steros wont let enough power get to the sub then what you want....